Optical square.



O. EPPENSTEIN.

OPTICAL SQUARE. APPLICATION IILED JULY 17,1813.

Patented Aug. 4, 1914.

hand.

onrrnn STATES PATENT OFFICE;

OTTO EPPENSTEIN OFQJENA; GERMANY, ASSIGNOR.TO THE FIRM 0F CARL ZEISS, OF a i JENA,,GERMA1 TY. i a

. 1 or'rIcaL i saunas.

means have become known, for lessening in optical squares consisting of two silvered glass plates fixed to an intermediate member those changes of form, which are caused by considerable differences of the mean temperature of the intermediate member on the one hand iand of the plates on the other The means there given consist in choosing for the intermediate member a material, which has approximately the same thermal conductivity as the glass plates. The patent specification 995235 specially suggests making the intermediate member also of glass, and makes this constructional form the subject of the second claim. As experience has shown, however, even those optical squares, in which glass serves as the material both for the reflecting plates and for the intermediate member, do not, under specially severe conditions, remain sufficiently free from changes of form. The cause of this is probably to be sought for in strains in the glass, which cannot be entirely equalized. The patent specification 995235 had believed it necessary to retain. glass as the material forthe reflecting plates, so that these might be silvered on their rear side and at the same time serve as a protection for the silvering. In consequence of such an arrangement it becamenecessary to make the thermal conductivity of the intermediate member conformable to that of the reflecting plates and to choose glass as the material for the intermediate member as well, if the reflecting plates and the intermediate member were to consist of the same material. a

The present invention has started from the knowledge, that the advantage, which with the use of glass consists in its transparency, is not to be regarded as of very great importance. Hence according to the present invention the above-mentioned advantage is waived and the front of the refleeting, plates is silvered, so that glass need no longer be used and the same thermal conductivity may be obtained for the re- Specification of Letters Patent. Patented Aug, 4,, 1914, Application filed July 17, 1913. Serial No. 779,487. 1

fleeting plates on the one hand and the intermediate member on the other by the employment of another material, which does not possess the defects inherent in glass. The 1 material in question may according to the" present. invention be any of those metals, the hardness of "which, 1 as: e.

great, to permit of the manufacture of ad equately accurate reflecting surfaces. In consequence of the comparatively high value of the thermal conductivity of the metals in such optical squares a rapid equalization of differences in temperature is made possible. Besides these metals marble, granite and other such minerals as are capable of taking a polish are suitable materials. Bodies made of porcelain or baked clay, inwhich those parts of the surface thatare to be silvered should be covered with a glazing, are particularlysuitable both on account of their condition as regards strain and because of their very low coeflicient of expansion.

When the silvering is to be protected from chemical or mechanical attacks, either each reflecting surface may be covered with a transparent layer of varnish or a glass plate or else the whole op ical square may be formed as a closed box provided with openings for the entrance and emergence of the light and the said openings closed by means of glass plates.

ppthat of steel" -=or of so called specular meta is sufficiently i In the annexed drawing five examples of optical squares accordlngto the mvention are shown in perspective. All five squares deflect by the angle between the reflecting sides being 45 in Figures lto 4 and 135 in Fig, 5.

In the optical. square, Fig. 1, the intermediate member a is a steel casing with three openings, the front one of which serves for the entrance and emergence of the light,

while the two lateral ones are closed by silvered steel plates 7) and 0, which are cemented in position. If the lower part of the hollow space of the optical square be rounded off, so as to correspond to a cylinder of rays, the broken axis of which is shown, and at the same time the top wall be omitted, there results the optical square according to Fig.2, which may, e. 9., consist of specular metal. The example Fig. 3 shows the optical square open at the bottom as Well. The intermediate member conmediate member a and the plates 6 and 0 are madeg' which plate areintegralwiththe intermediate member .1 and are silvered on vtheir front sides. The optical square acfi id llg :t sFig- 1 is m d to b ofpersww lain oribaked clay The intermediate meme erilth yplate r ei intesrah w The. re glaz ed on their front. sides and;

. P a e 15 thensilvered Copies otth is patent niay Beo btai ned; 101'.

l claim:

' 1., Optical square consisting of two reiieet. 5 ing plates land an intermediatemember, all. th-ree oi the same material, the plates being fixed t o the intermediate member v and sil- L v'ered on their front; sides.

111 m; qaemty addressing. the; 6 Commissioner on-1R0," i

'2. Optical square consisting of two refleeting plates and an intermediate-member,

w a1-l;;three 0f ;metal-,- -t he plates being fixed to the intermediate member and silvered on 25 their front sides. p I; p v OTIO' EPPENSTEIN; Witnes$esz H f 5 RAU KR -Q R FRITZ Sal-men. 

